Typically, in a dry process of cement manufacture, cement clinker production unit comprises a preheater wherein the raw meal is preheated, a precalciner wherein the preheated raw meal is partially, around 90%, calcined to transform the CaCO3 into CaO and CO2, a kiln wherein the meal is fully calcined and sintered to form the clinker, and a clinker cooler. The precalciner and the kiln are supplied with fuel and the preheater stage is heated by hot flue gases coming from the precalciner and from the kiln. Moreover, air heated in the cement clinker cooler is supplied to the kiln and to the precalciner as combustion air. At the best, the design specific heat consumption of the aforementioned process is around 690 kcal per kilogram of clinker while the theoretical heat consumption is around 420.
Thus, in cement clinker production, the energy efficiency is limited by two factors:
i) the gases entering the preheater have more heat than needed to preheat efficiently the raw material, as the flue gas flow is increased around 50% by the carbon dioxide released and since the most endothermic reaction: calcination, only occurs in theory above 800° C. but closer to 900° C. in practice; and
ii) the produced clinker needs more air to cool than that needed for efficient combustion, so there is an energy loss through the cement clinker cooler.
Moreover, cement manufacturing process involves a very high electrical consumption (around 120 kWh per ton of cement or 150 kWh per ton of clinker) related to the raw meal preparation (crushing, milling, transportation), due to the use of very large air fans (transportation, classification, clinker cooling) and cement grinders, for instance.
In order to increase the energy efficiency of such installation, it is known to use cogeneration technology. Usually, exit flue gases from the preheater stage receiving the raw meal and/or the excess hot air from the clinker cooler are led to heat recovery steam generators, and an electric generating device comprising a steam turbine with an electrical generator is driven by the produced steam. U.S. Pat. No. 6,749,681 describes a method for producing electricity and calcined raw mix in a circulating fluidized bed reactor and illustrates the use of the calcined raw mix to produce clinker. However, the technologies of circulating fluidized bed reactor are expensive and therefore, little-used for cement clinker production.
U.S. Pat. No. 4,052,148 discloses a cement clinker production unit wherein fuel is injected to a steam generator and flue gases discharged from the steam generator are sent to the cement clinker production process. Alternatively, flue gases bypassed from the cement clinker production process are sent to the steam generator as combustion air. The aim of this method is to decrease fuel consumption in the cement clinker production and to produce electrical energy. However, this document does not teach supplying more fuel to the precalciner and/or to the kiln than needed for the cement clinker production process.
Therefore, in the design case of 690 kcal per kg of clinker, around 1.7 tons of gases per ton of clinker (of which around 0.6 is the CO2 released) leave the preheaters at a temperature of around 293° C. and the average electrical energy produced by usual cogeneration systems is around 15 kWh per ton of clinker. The electrical energy that can be produced by the excess hot air from the clinker cooler is about the same, so altogether 30 kWh per ton of clinker can be produced with this specific heat consumption. Since the average consumption of the cement production process is 150 kWh/Ton of clinker, cogeneration covers only 20% of the consumption.
Moreover, although the power industry has moved towards more efficient combined cycles, electricity remains an expensive commodity due to the natural gas price increase owing to its non-renewable characteristic. Efforts have been made to use negative-cost or less-expensive fuels, such as wastes and low grade alternative fuels, by gasification, fluidized beds and other technologies, but these technologies are expensive. To respond to the energy price increase, the cement industry becomes more fuel energy effective and the fuel consumption per kg of produced clinker decreases.
Furthermore, the cement industry partly switches fuels to low-grade alternative fuels and wastes without the use of the aforementioned technologies required by the power industry. It turns out that the cement clinker production process works like a natural fluidized bed due to its highly alkaline environment and the long residence time, with the additional advantages of its high temperature, and that the solid waste is safely incorporated into the cement. Nevertheless, the use of high humidity and/or low heating value fuels and waste in the cement clinker production, combined with the high excess air usually needed to burn such fuels, increases the volume of hot flue gases and decreases efficiency, but still with economic and environmental benefits from the fuel switching. Under these conditions the flow of gases leaving the preheaters increases as well as its temperature, raising the cogeneration capability. A typical value of the cogeneration of the preheater and the clinker cooler combined is 40 kWh/Ton of clinker, which covers 27% of the average consumption of the cement production process of 150 kWh/Ton of clinker. However, the electrical needs of the cement production process remain significant and this kind of cogeneration does not take full advantage of the fact that the cement clinker production process is a natural substitute of an expensive fluidized bed.